The TUC's leader is asking for government action to avert "mass unemployment" amid the pandemic.
Trades Union Congress general secretary Frances O'Grady has told its annual meeting the chancellor must "stand by working families" as the furlough scheme nears its end.
She said: "If the government doesn't act, we face a tsunami of job losses."
A government spokesperson said supporting jobs was "an absolute priority".
Under the government's furlough scheme, workers placed on leave have been able to receive 80% of their pay, up to a maximum of £2,500 a month.
Take-up has been significant, with 9.6 million workers furloughed since March.
The scheme is due to finish at the end of October and Chancellor Rishi Sunak has repeatedly ruled out an extension to it.
Speaking at the trade union body's congress in London on Monday, Ms O'Grady said time is running out to prevent huge job losses as the job retention scheme (JRS) comes to a close: "Millions of livelihoods were saved [by the scheme] - both employees and the self-employed. From this Thursday, it will be just 45 days before the JRS ends.
"That's the notice period that companies have to give if they intend to make mass redundancies.
"Rishi Sunak, stand by working families - don't walk away. It's so much better to keep people working, paying their taxes, spending and helping to rebuild the economy."
The trade unions body is also calling for a new "job protection and skills deal", which would include mandatory training and "up-skilling" for workers placed on furlough, for example.
A government spokesperson said: "Supporting jobs is an absolute priority which is why we've set out a comprehensive 'Plan for Jobs' to protect, create and support jobs across the UK by providing significant, targeted support where it is needed the most.
"We are continuing to support livelihoods and incomes through our £2bn Kickstart scheme, creating incentives for training and apprenticeships, a £1,000 retention bonus for businesses that can bring furloughed employees back to work, and doubling the number of frontline work coaches to help people find work.
"We are also supporting and protecting jobs in the tourism and hospitality sectors through our VAT cut and last month's Eat Out to Help Out scheme."